Book excerpt: A mood is defined as the prevailing psychological state (habitual or relatively temporary). It is further defined as a feeling, state or prolonged emotion that influences the whole of one's psychic life. It can relate to passion or feeling; humour; as, a melancholy mood; a suppliant mood. Mood can and does affect perceived health, personal confidence, one's perceptions of the world around us and our actions based on those perceptions. Moods can and do change often although mood swings of a sharp nature may be a symptom of underlying disease. Moods may signify happiness, anger, tension, or anxiety. Chronic periods of any mood state may be an indicator of a disorder as well. This new book gathers important research from throughout the world in this rapidly changing field.